The present application relates to a cathode active material, a cathode, and a non-aqueous electrolyte secondary battery and, more particularly, to a cathode active material containing a composite oxide containing, for example, lithium and a transition metal, a cathode, and a non-aqueous electrolyte secondary battery.
In recent years, techniques of portable electronic apparatuses have remarkably been developed and electronic apparatuses such as cellular phones, notebook-sized computers, and the like have been started to be recognized as fundamental techniques which support the advanced information world. Researches and developments regarding the realization of advanced functions of those electronic apparatuses are vigorously being progressed. Electric power consumption of those electronic apparatuses is also increasing more and more in proportion to the realization of the advanced functions. On the contrary, it is demanded that those electronic apparatuses can be driven for a long time and it is inevitably demanded to realize a high energy density of a secondary battery as a driving power source. It is also demanded to extend a cycle life in consideration of an environment.
The higher energy density of the battery is desirable from viewpoints of an occupied volume, a mass, and the like of the battery built in the electronic apparatus. At present, lithium ion secondary batteries have been built in most of the apparatuses because they have higher voltages and higher energy densities than those of other battery systems.
Ordinarily, in the lithium ion secondary battery, a lithium cobalt acid is used for a cathode, a carbon material is used for an anode, and an operating voltage is set to a value within a range from 4.2V to 2.5V. In a unit cell, a terminal voltage can be raised to 4.2V owing to an excellent electrochemical stability of a non-aqueous electrolyte material, a separator, and the like.
Many examinations are being progressed in order to realize further advanced performance and an enlargement of application of such a lithium ion secondary battery. As one of the examinations, for example, it is being examined to raise an energy density of a cathode active material including the lithium cobalt acid by a method of raising a charge voltage or the like, thereby realizing a large capacitance of the lithium ion secondary battery.
However, there is such a problem that when the charge and discharge are repeated at a large capacitance, a capacitance deterioration occurs and a battery life is shortened. There is also such a problem that when the battery is used under a high temperature environment, an increase in internal resistance of the battery progresses, it is difficult to extract a sufficient battery capacitance, or the like. Therefore, hitherto, a method of reforming the cathode active material by mixing a small amount of LiMn1/3Co1/3Ni1/3O2 or the like to the cathode active material and using such a material or by coating the surface with another material has been used.
For example, a method of improving cycle characteristics by coating the surface of a cathode electrode with a metal oxide has been disclosed in Patent Document 1 (Japanese Patent No. 3172388). A method of raising a thermal stability by coating the surface of a cathode active material with a metal oxide has been disclosed in Patent Document 2 (Japanese Patent No. 3691279).
Upon coating of the surface of the cathode active material, effects of improving the cycle characteristics and improving the thermal stability by the coating form have also been examined. For example, methods of uniformly coating a lithium transition metal composite oxide have been disclosed in Patent Document 3 (JP-A-7-235292), Patent Document 4 (JP-A-2000-149950), Patent Document 5 (JP-A-2000-156227), Patent Document 6 (JP-A-2000-164214), Patent Document 7 (JP-A-2000-195517), and Patent Document 8 (JP-A-2002-231227). A cathode active material in which a lump of a metal oxide has been deposited on a metal oxide layer has been disclosed in Patent Document 9 (JP-A-2001-256979).
Elements which are used to coat the surface have also been examined. For example, a cathode active material in which one or more surface processing layers containing two or more coating elements have been formed on the surface of a lithium compound serving as a core has been disclosed in Patent Document 10 (JP-A-2002-164053).
A cathode active material in which a coating film made of metal fluoride has been formed on the particle surface has been disclosed in Patent Document 11 (Japanese Patent No. 3157413). A cathode active material in which the particle surface has been coated with crystalline metal fluoride has been disclosed in Patent Document 12 (Japanese Patent No. 3141858).
Further, a technique of specifying an XPS (X-ray Photoelectron Spectroscopy) energy value of fluorine on the particle surface has been disclosed in Patent Document 13 (JP-A-2003-221235).